If I understand correctly you have the engine bolted to the transmission but can't turn the convertor to line up holes with the flex plate.

Is the back of the block flush against the transmission bell housing or is there a space there? Were you careful to make sure the convertor pump drive hub was properly seated into the front pump? If it is, you should have enough clearance for the convertor to spin without a problem.

The front hub on the convertor has to nest into the opening on the crank flange. If it's cocked at all you'll have trouble. I don't think the balance weight would be the problem. I ran into this problem putting a 360/727 in a b body this fall. The convertor was new, and I'd double checked everything before bolting them together, but still had a problem. Loosened up the bell housing bolts and wiggled things a bit and it all lined up properly.

Not sure if this helps or not. Best of luck.